In recent years, instead of a method of producing neutron beams with high energy efficiency as used in a large facility, there has been tried to be developed a method of producing neutron beams with use of low energy beams. In such a method, neutron beams are produced by, for example, irradiating a target (e.g., Be, Li, or the like) with proton beams to thereby cause a nuclear reaction. This method can produce neutron beams with use of extremely low energy proton beams.
According to the above method, for example, there is no need to provide a gigantic radiation blocking structure which can be accepted only in a large facility. It is therefore considered that a neutron source employing the above method is extremely suitable for use in a small facility. In particular, in a case where proton beams having an energy of not more than 13 MeV are used, the neutron source can be easily handled because an amount of a resultant radioactivated material is extremely low.
However, low energy proton beams penetrate a target extremely shallowly. Therefore, a proton, with which a material is irradiated becomes hydrogen and the hydrogen is easily accumulated in the target locally. In view of this, it is known that a target is broken mainly by a mechanism of hydrogen embrittlement for an extremely short time. This phenomenon is called blistering. From a practical standpoint, the blistering is a fatal problem in a low energy neutron generator employing the above method.
In view of this problem, various researches have been conducted. There is reported a neutron source for producing neutron with use of a Li (p, n) reaction in which Li is used (Non-patent Literatures 1-4).
In Non-patent Literatures 1 through 3, blistering of a Li target was verified. Specifically, Non-patent Literatures 1 through 3 report that, in a case where a Li target is irradiated with proton beams of 2.5 MeV or 1.9 MeV, blistering occurs due to a beam current having 10 mA after 3.5 hours from this irradiation. Those literatures conclude that the blistering is not problematic from a practical standpoint, because a single irradiation time period in a BNCT therapy (Boron Neutron Capture Therapy) is shorter than the above time period.
Non-patent Literature 4 reports a structure for preventing hydrogen embrittlement of a target. According to the report, protons (hydrogen atoms) which have passed through Li are absorbed and diffused by the structure in which a thin film made from Pd having high hydrogen transparency is formed under Li.
Non-patent Literature 5 shows a result of simulation for preventing hydrogen embrittlement with use of a target other than Li. From the simulation, such a result was obtained that, when a neutron source is formed by joining thin Be and Nb, hydrogen embrittlement is preventable because almost all irradiated proton beams penetrate Be and are remained in Nb. Therefore, there is a possibility that the structure stably prevents hydrogen embrittlement of a neutron source for a long time.
Non-patent Literature 6 reports results of tests regarding conditions for causing blistering in various metals when the various metals are irradiated with proton beams. The test is carried out by observing the metals which have been irradiated with proton beams of 200 keV with use of, for example, an electron microscope by an optical method. As the result of this, it is reported that blistering does not occur in V and Ta under the tested conditions.